Confined-Space Combustion Air After Weatherization
A 1960s bungalow has a 50,000 Btu/h natural-draft water heater and an 80,000 Btu/h natural-draft furnace side by side in an 8 ft by 10 ft room with an 8 ft ceiling. Both vent into a shared masonry chimney on an exterior wall. The room has a solid door with one 12 in. by 12 in. metal grille near the floor; the grille has 75% free area. After the homeowner installs new windows and spray foam, the water heater spills flue gas at the draft hood whenever the furnace starts. A CO detector in the hallway reads 35 ppm. The technician measures 7 in. w.c. supply pressure and sees a clean blue burner flame. The chimney has no liner. The homeowner asks whether opening the door will fix it and whether the CO detector is enough.
1. Calculate whether the room is confined and whether the existing grille meets combustion-air requirements. Show your numbers.
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Room volume is 8 × 10 × 8 = 640 ft³. Total input is 50,000 + 80,000 = 130,000 Btu/h. The usual unconfined-space threshold is 50 ft³ per 1,000 Btu/h, so this room would need 130 × 50 = 6,500 ft³. At 640 ft³ it is a confined space. For two permanent openings, each needs 1 in² free area per 1,000 Btu/h, or 130 in² each, with a 100 in² minimum per opening. The existing grille is 12 × 12 = 144 in², but at 75% free area it provides only 108 in², and it is only one low opening. It fails on both free area and the required high/low arrangement. Opening the door may temporarily change the reading but is not a permanent code-compliant air supply.
2. What is the most likely cause of spillage, and what diagnostic steps would you take before adding air?
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Spillage when the furnace starts points to depressurization or a venting/draft problem, not simply a dirty burner. Check the chimney for blockage, nesting, collapse, or missing liner; check the vent connector for correct rise and slope toward the chimney; perform a draft and spillage test at the water heater draft hood with the door closed and the furnace operating. Measure pressure in the room and look for competing exhaust: kitchen fan, bath fan, dryer, or the furnace itself backdrafting the shared flue. Use a smoke or match test and a manometer. An exterior unlined chimney can be cold and oversized, so it may never develop reliable draft. Do not add a grille and leave without finding why the flue gas is entering the room.
3. Recommend permanent corrections and explain why a CO detector alone is not a solution.
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Provide two permanent openings sized to the code, or a direct outdoor-air duct designed for the total input; correct the vent connector rise and slope; and reline, repair, or replace the chimney as needed. If the shared flue cannot be made safe, separate the vents or install a power-vented or high-efficiency appliance. Address depressurization by relocating or interlocking large exhaust fans and by making sure the room can draw combustion air. A CO detector is a backup warning device, not a prevention method: it does not stop spillage, and it may not alarm before occupants are exposed. Verify draft, spillage, and CO after the repair, and document the before-and-after readings.